1. FIELD OF THE INVENTION
This invention relates to the structure of a light-receiving face which can be employed for photosensors that are operated in the storage mode, more concretely, for a photoconductive target of an image tube, a solid-state imager, etc.
2. DESCRIPTION OF THE PRIOR ART
As a typical example of the photosensor which has heretofore been used in the storage mode, there is a photoconductive type image tube shown in FIG. 1. It is made up of a light-transmitting substrate 1 which is usually called the "face plate", a transparent conductive layer 2, a photoconductive layer 3, an electron gun 4, and an envelope 5. An optical image formed on the photoconductive layer 3 through the face plate 1 is photoelectrically converted, and is stored as a charge pattern in the surface of the photoconductive layer 3. The charge pattern is read in time sequence by a scanning electron beam 6.
At this time, an important property required for the photoconductive layer 3 is that the charge pattern does not decay due to diffusion within a time interval in which a specified picture element is scanned by the scanning electron beam 6 (that is, a storage time). Accordingly, semiconductors whose resistivities are not lower than 10.sup.10 .OMEGA..multidot.cm, for example, chalcogenide glasses containing Sb.sub.2 S.sub.3, PbO and Se are ordinarily employed as the materials of the photoconductive layer 3. In case where a material such as Si single crystal whose resistivity is lower than 10.sup.10 .OMEGA..multidot.cm is employed, the surface of the layer 3 on the electron beam scanning side needs to be divided in a mosaic fashion so as to prevent the decay of the charge pattern. Among these materials, the Si single crystal is complicated in the working process. The high-resistance semiconductors usually contain high densities of trap levels hampering the traveling of photo carriers. Therefore, they are inferior in the photo response and are liable to cause the drawback that a long decay lag and an after-image develop as the imaging device.
This invention intends to eliminate the above disadvantages.